The portion of a human extremity that remains after amputation is commonly referred to as an amputation stump or residuum. In the following disclosure the expression residuum is used with reference both to an amputated arm or an amputated leg. A prosthetic device, such as a leg prosthesis with or without a knee joint or an arm prosthesis with or without an elbow joint, may occasionally be attached to the residuum. A sleeve element that is carried about the amputated limb has a first member of a lock mechanism in the distal end of the sleeve. A second member of the lock mechanism is arranged on the prosthetic device for detachable connection of the prosthesis and the residuum.
In prior lock mechanisms the first member of the mechanism comprises a rigid locking pin, centrally arranged in the distal end of a flexible sleeve element that encompasses the end of the residuum. When connecting the prosthesis, the locking pin, usually made of steel, is inserted in a seat formed in the second member of the lock mechanism. The seat for the locking pin is formed in the bottom of a rigid socket element, adapted to the shape of the residuum. The prosthesis is carried on the rigid socket, and the socket transferring loads from the residuum to the prosthetic device via the lock mechanism.
A typical lock mechanism of this kind is disclosed in U.S. Pat. No. 5,507,837 (LAGHI), comprising a detachable, rigid locking pin that has external ribs and is arrested in the prosthesis by means of a toothed wheel which rotates in one direction only. By insertion of the locking pin, the toothed wheel is brought into rotation through the teeth engaging the ribs on the locking pin. Upon detachment of the locking pin, the toothed wheel is displaced from engagement with the ribs by manually applying a pressure that moves the toothed wheel to the side, transversely from the axis of the locking pin. Other examples of prior lock mechanisms for the same purpose may be found in U.S. Pat. No. 5,226,918 (SILAGY et al.), U.S. Pat. No. 5,888,234 (LITTIG et al.), U.S. Pat. No. 6,051,026 (PIRO et al.), e.g.
Previous lock mechanisms suffer from drawbacks that may occasionally cause discomfort for the wearer of the prosthesis. The present invention seeks to eliminate these drawbacks, drawbacks which may cause problems as will be more closely explained below.
The rigid and individually adapted socket, carrying the prosthesis, is formed as an inverted copy of the residuum. Since the shape of the residuum is strongly individual there is a problem, when adapting the socket, simultaneously to ensure that the locking pin's seat in the bottom of the socket is correctly centered and aligned with the locking pin. When dressing the soft sleeve onto the residuum there is also a risk that the locking pin in the distal end of the sleeve is positioned at the side of an ideal insertion direction relative to the seat, and/or at a slanting angle relative to the insert direction. In such case insertion of the locking pin becomes complicated, and discomforting pressure and tension loads will be transferred to the residuum that is connected to the prosthesis.
Another drawback associated with some of the conventional lock mechanisms is the relative building height. The height dimension is the result from an exaggerated length of the locking pin, the pin being used for pulling in the residuum into the rigid socket by operation of the lock mechanism. This functionality may be seen as an attempt to meet the problem of a tilted and wrongly aligned locking pin that is difficult to insert in the seat of the lock mechanism. However, this solution does not eliminate the problem of undesired loads acting on the residuum and caused by the locking pin deviating from an ideal insert direction. An exaggerated building height may also cause difficulties in adapting the prosthesis to the length of the residuum.
Yet another drawback related to some known lock mechanisms may be referred to the axial stiffness of the locking pin. Walking with a leg prosthesis causes the muscles and other soft tissue of the residuum to move in a pumping motion, sometimes leading to chafing caused by the rigid socket that encloses the residuum. This so called piston effect is made worse through the locking pin which has no axial elasticity. The combination of a long locking pin and radial forces generated from an inaccurate alignment relative to the seat may also cause the locking pin to be jammed in the seat, requiring a technician for removing the prosthesis.